This invention relates to cluster beams, and, more specifically, to apparatus for producing cluster beams using a cold cathode ionizer.
The deposition of thin films upon substrates is an important manufacturing and research tool in a variety of fields. For example, microelectronic devices are prepared by depositing successive film layers onto a substrate to obtain specific electronic properties of the composite. Photosensitive devices such as vidicons and solar cells are manufactured by depositing films of photosensitive materials onto substrates. Optical properties of lenses are improved by depositing films onto their surfaces. These examples are, of course, only illustrative of the thousands of applications of thin film deposition techniques.
In the highly controlled approach to thin-film deposition that is characteristic of applications wherein a high quality film is required, the film is built up by successive deposition of monolayers of the film, each layer being one atom thick. The mechanics of the deposition process can best be considered in atomistic terms. Generally, in such a process the surface of the substrate must be carefully cleaned, since minor contaminant masses or even contaminant atoms can significantly impede the deposition of the required highly perfect film. The material of the film is then deposited by one of many techniques developed for various applications, such as vapor deposition, sputtering, chemical vapor deposition, or electron beam evaporation.
In another technique for depositing thin films, ionized clusters of atoms are formed in a cluster source. These clusters usually have on the order of about 1000 (and sometimes up to 10,000) atoms per cluster. The clusters are ionized and then accelerated toward the substrate target by an electrical potential that imparts an energy to the cluster equal to the accelerating voltage times the ionization level of the cluster. Upon reaching the surface of the substrate target, the clusters disintegrate at impact. Each atom fragment remaining after disintegration has an energy equal to the total energy of the cluster divided by the number of atoms in the cluster. The cluster prior to disintegration therefore has a relatively high mass and energy, while each atom remaining after disintegration has a relatively low mass and energy. The energy of the atom deposited upon the surface gives it mobility on the surface, so that it can move to imperfections such as kinks or holes that might be present on the surface. Some of the deposited atoms come to rest in the imperfections, thereby removing the imperfections and increasing the perfection and density of the film. Other approaches to using clusters have been developed, and it appears that deposition using cluster beams is a promising commercial film-manufacturing technique.
In current apparatus for producing cluster beams, the clusters are ionized by a thermionic ionizer. Such an ionizer includes a cathode that is heated to a very high temperature by the passage of an electrical current therethrough. The hot cathode emits electrons, which are then accelerated toward and through the beam of clusters by an anode. Thermionic ionizers are operable for their purpose, but have significant drawbacks. The most important of the drawbacks is their short lifetimes in some applications. If the beam emitted by the source contains reactive species such as oxygen or chlorine, they may quickly attack the heated cathode and cause it to fail. Also, the positioning of the thermionic ionizer in the most optimum location, close to the cluster source, may result in periodic electrical breakdowns as a result of the interaction of the thermionic electrons and the unclustered gas atoms in the beam.
Accordingly, there is a need for an improved apparatus for producing cluster beams that is less susceptible to degradation by reactive species. The present invention fulfills this need, and further provides related advantages.